1. Field of the Invention
The present invention relates to a dielectric film structure having excellent dielectric property, piezoelectric property, pyroelectric property and ferroelectric property and available as an element such as an actuator, a memory or a sensor. Further, the present invention relates to a piezoelectric actuator having such a dielectric film structure. Furthermore, the present invention relates to an ink jet head utilizing such a piezoelectric actuator as a discharge pressure generating element for discharging liquid.
2. Related Background Art
Presently, a dielectric film structure having a dielectric film including solid solution of lead titanate zirconate (Pb(Zr, Ti)O3, PZT), barium titanate (BaTiO3), lead titanate zirconate-lead magnesium niobate (Pb(Zr, Ti)O3—Pb(Mg1/3Nb2/3)O3, PZT-PMN) has widely been used for various elements such as actuators, memories and sensors by utilizing dielectric, piezoelectric, pyroelectric and ferroelectric properties thereof. At present, as electronic/electric equipment has been made compact and made to have a high function, there has been highly requested for compactness and a high integrating ability of the element itself.
For example, a piezoelectric actuator having the dielectric film structure has been utilized as ink discharging means of an ink jet printer which has been popularized as an output device of a personal computer.
Various types of ink jet heads utilizing the piezoelectric actuator are known. As such types, for example, there are a bi-morph type disclosed in Japanese Patent Publication No. S53-12138 (1978), a uni-morph type disclosed in Japanese Patent Publication No. S62-22790 (1987) and a share mode type disclosed in U.S. Pat. No. 4,584,590, Japanese Patent Publication No. H7-33089 (1995) and U.S. Pat. No. 5,265,315.
Now, an ink jet head will be explained with reference to an ink jet head using a piezoelectric actuator of uni-morph type. FIG. 4A is a perspective view of an ink jet head of uni-morph type and FIG. 4B is a sectional view taken along the line 4B-4B in FIG. 4A. The ink jet head mainly formed from three parts comprising a piezoelectric actuator 8, a flow path substrate 9 and a nozzle plate 10. A plurality of nozzles 3 are formed in the nozzle plate at predetermined pitch and pattern and each nozzle forms a discharge port at a back side of the flow path substrate, which is an opening through which ink is discharged. Further, the nozzles correspond to respective actuator and ink pressurizing chambers 1 are formed in the flow path substrate in correspondence to the respective nozzles.
The nozzle plate and the flow path substrate are joined together so that the nozzles are communicated with the corresponding ink pressurizing chambers. Further, a common ink chamber 2 is formed in the flow path substrate and all plural ink pressurizing chambers are communicated with the common ink chamber. One (ceiling portion in the drawings) of side walls of the ink pressurizing chamber 1 also acts as a vibrating plate 4 of the piezoelectric actuator. A lower electrode 5, a dielectric film 6 and an upper electrode 7 are laminated in order on the vibrating plate, thereby constituting the piezoelectric actuator.
When the ink is discharged, the ink is supplied from the common ink chamber 2 to the ink pressurizing chamber 1 thereby to fill the ink pressurizing chamber 1. In this condition, by applying voltage to the upper electrode 7 and the lower electrode 5 of the piezoelectric actuator 8, the dielectric film 6 can be contracted or expanded in a direction parallel to a surface of the film. As a result, the piezoelectric actuator 8 is flexed toward inside or outside of the ink pressurizing chamber 1. For example, when the piezoelectric actuator is flexed toward the inside of the ink pressurizing chamber, the ink in the ink pressurizing chamber is pressurized, so that an ink droplet can be discharged from the nozzle by the pressure.
The dielectric film of the ink jet head which has presently been popularized is normally formed by screen printing of piezoelectric ceramic paste. The piezoelectric ceramic paste is obtained by adding a small amount of organic binder to raw material powder of piezoelectric material. The piezoelectric ceramic paste is patterning-coated by screen printing on the lower electrode in correspondence to the ink pressurizing chambers formed at the predetermined pitch and, thereafter, sintering is performed, thereby obtaining a multi crystal dielectric film.
However, in consideration of development of high density of nozzle arrangement and lengthening of the ink jet head (increase in the number of nozzles per one head), the method using the screen printing has the following problems.
In a case where the high density of the nozzle arrangement in the ink jet head is sought, a width of the ink pressurizing chamber must be decreased accordingly and greater displacement of the piezoelectric actuator is required. Accordingly, in order to achieve higher density of the nozzle arrangement, the dielectric film obtained by the screen printing may not provide a satisfactory piezoelectric property.
Further, in the formation of the dielectric film by using the screen printing, in order to perform the sintering, a temperature range from 950° C. to 1300° C. and a time range from several ten minutes to several hours are required. A pattern of the dielectric film so obtained causes positional deviation from a pattern obtained at a time when the screen printing was performed due to volume contraction during the sintering. Such positional deviation of the dielectric film may cause a great problem regarding influence upon a discharging property as higher density of the nozzle arrangement is achieved and the length of the ink jet head is increased. Further, in general, although minute working of a silicon substrate using a semiconductor manufacturing technique is considered to be bright as means for achieving the high integrating ability of the ink pressurizing chambers, in a case where the dielectric film is formed by the screen printing, deterioration of the silicon substrate due to the heating during the sintering may not be avoided.
In order to solve these problems, there has been developed a technique in which a temperature of a film forming condition is reduced by using a method such as a spattering method, a sol-gel method or a metal organic chemical vapor deposition (MOCVD) method and the properties are improved by controlling a crystal structure of the dielectric film.
For example, Japanese Patent Application Laid-open No. H6-350154 (1994) discloses a dielectric (piezoelectric) film element in which lead titanate zirconate of rhombic crystalline system having (111) orientation greater than 70% is film-formed on a substrate by a spattering method and a dielectric (piezoelectric) film element in which lead titanate zirconate of square crystalline system having (001) orientation greater than 70% is film-formed on a substrate by a spattering method and teaches usage as a liquid jetting device.
However, the dielectric film (piezoelectric) elements disclosed in this document have several problems.
First of all, although the piezoelectric property thereof is improved in comparison with that of a sintering element of multi crystal lead titanate zirconate (PZT), in many cases, such piezoelectric property may be insufficient to realize an ink jet printer having high density nozzle arrangement. Further, endurance of the dielectric film element is smaller than that of a multi crystal PZT sintering element and, if the dielectric film element is used repeatedly, the piezoelectric property thereof may be reduced considerably.